is-type [level-1 | level-1-2 | level-2-only]
level-1 (Optional) Router performs only Level 1 (intra-area) routing. This router learns only about destinations inside its area. Level 2 (interarea) routing is performed by the closest Level 1-2 router. level-1-2 (Optional) Router performs both Level 1 and Level 2 routing. This router runs two instances of the routing process. It has one link-state packet database (LSDB) for destinations inside the area (Level 1 routing) and runs a shortest path first (SPF) calculation to discover the area topology. It also has another LSDB with link-state packets (LSPs) of all other backbone (Level 2) routers, and runs another SPF calculation to discover the topology of the backbone, and the existence of all other areas. level-2-only (Optional) Routing process acts as a Level 2 (interarea) router only. This router is part of the backbone, and does not communicate with Level 1-only routers in its own area.
To configure the routing level for an instance of the IS-IS routing process, use the is-type command in router configuration mode. To reset the default value, use the no form of this command.
In conventional IS-IS configurations, the router acts as both a Level 1 (intra-area) and a Level 2 (interarea) router.
In multiarea IS-IS configurations, the first instance of the IS-IS routing process configured is by default a Level 1-2 (intra-area and interarea) router. The remaining instances of the IS-IS process configured by default are Level 1 routers.
We highly recommend that you configure the type of IS-IS routing process. If you are configuring multiarea IS-IS, you must configure the type of the router, or allow it to be configured by default. By default, the first instance of the IS-IS routing process that you configure using the router isis command is a Level 1-2 router.
If only one area is in the network, there is no need to run both Level 1 and Level 2 routing algorithms. If IS-IS is used for Connectionless Network Service (CLNS) routing (and there is only one area), Level 1 only must be used everywhere. If IS-IS is used for IP routing only (and there is only one area), you can run Level 2 only everywhere. Areas you add after the Level 1-2 area exists are by default Level 1 areas.
If the router instance has been configured for Level 1-2 (the default for the first instance of the IS-IS routing process in a Cisco device), you can remove Level 2 (interarea) routing for the area using the is-type command. You can also use the is-type command to configure Level 2 routing for an area, but it must be the only instance of the IS-IS routing process configured for Level 2 on the Cisco device.
The following example specifies an area router:Router(config)#router isis Router(config-router)# is-type level-2-only
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